<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
</head>
<body>
<div id="app">
  <h1>{{message}}</h1>
</div>

<script src="./js/vue.js"></script>

<script>
  const app = new Vue({
    el:'#app',
    data:{
      message:'Hello World'
    },
    watch:{
      message(newValue){
        newValue="哈哈哈";
        this.message = newValue;
      }
    }
  });
</script>
</body>
</html>